A general multi-antenna device is made by arranging a plurality of antennas with the same characteristics at half-wavelength intervals to form beams. The reason of this arrangement is that pattern similarity and physically-short distance between antennas with the same characteristics cause channel characteristics to be similar if they are arranged at intervals not greater than a half-wavelength.
In this case, as a plurality of channels has characteristics similar to each other, it is impossible to implement good characteristics, for example, multiplexing obtained from multiple antennas or robustness against multi-path fading.
Because of the aforementioned disadvantages, there is proposed MIMO (Multiple Input Multiple Output) communication that is carried out by using a plurality of the same or dual-polarized antennas arranged at half-wavelength intervals. However, a disadvantage involved in this arrangement is that it is essential to secure a large space for the antennas because they are arranged at half-wavelength intervals.
Furthermore, since it is necessary to place tens to hundreds of antennas for the ultra-multi-antenna technology, for example, massive MIMO, recently studied, the space for the antennas must be secured.
An antenna radiation pattern f(θ,Φ) is defined with the following Equation 1 by using the spherical vector waves mode, orthogonal to each mode:
                              f          ⁡                      (                          θ              ,              Φ                        )                          =                              ∑                          α              =              0                        ∞                    ⁢                                    c              α                        ⁢                                          A                α                            ⁡                              (                                  θ                  ,                  Φ                                )                                                                        [                  Equation          ⁢                                          ⁢          1                ]            
where Aα(θ,Φ) represents a spherical vector wave mode, and cα is the coefficient of a radiation pattern for each spherical vector wave mode.
The MIMO system that employs a conventional multi-antenna device uses just two of the spherical vector wave modes to increase channel capacity because dual-polarization dipole antennas are integrated in the MIMO system.
However, the conventional system gives rise to a problem that pattern/polarization gains are not effectively obtained because it uses just two spherical vector wave modes.